1. Field of the Invention
The present invention relates to an apparatus for discharging droplets, and particularly to a droplet discharge apparatus which prevents defective discharge due to drying or solidification of a composition to be discharged.
2. Description of the Related Art
In recent years, research on directly drawing a circuit pattern over a substrate using a semiconductor material, a metal material, or an insulating material, so-called direct drawing, has been conducted actively. It is specifically intended that an electronic circuit be drawn and an electronic device be printed using an apparatus having a droplet discharging means, as if characters are printed out by a printer.
A conventional method for manufacturing an electronic device includes a considerably complicated process in forming a pattern of a circuit or the like. For example, in the case of forming one wiring, a film of a semiconductor material or a metal material to be a wiring is formed over a substrate; a region of the film to be used for the wiring is specified and provided with a resist or the like; lastly, the other part of the film is removed by etching or the like while leaving the specified region unremoved. This process is repeated to form a desired circuit pattern. However, such a process has a large number of steps and a step of forming or removing a film needs to be performed in vacuum. Therefore, heavy use of a huge expensive vacuum apparatus increases manufacturing cost.
On the other hand, in the case of using the direct drawing, a circuit pattern such as a wiring can be directly formed. Thus, a conventional three-stage process can be replaced by a one-stage process. Since the circuit pattern can be manufactured under atmospheric pressure, and a vacuum apparatus is unnecessary, drastic cost reduction can be achieved. Therefore, direct drawing with the use of a droplet discharge apparatus attracts considerable attention.
The above-described direct drawing is performed using, for example, a droplet discharge apparatus such as an inkjet apparatus. But, in the case of discharging a high-viscosity material or the like by a droplet discharge apparatus, defective discharge caused by drying of the material or the like has become a problem. Therefore, various solutions have been searched for in order to solve the defective discharge.
For example, there is a method in which a head having a discharge opening is wiped with a solvent such as an organic solvent or a discharge solution itself before discharging a material, a method in which an entire head is capped and then soaked into a solvent, or a method in which a head is placed in space filled with solvent vapor. Alternatively, there is also a method in which a moisturizing agent is added to a liquid to be discharged or a meniscus control method in which drying or solidification is prevented by vibrating and agitating a liquid with pulse voltage, not so high as to discharge a liquid, applied to a piezoelectric element (for example, Reference 1: Japanese Patent Application Laid-Open No. 9-290505).
Although the method in which a head is wiped before discharging a material, an entire head is capped and then soaked into a solvent, or a head is placed in space filled with vapor is effective against defective discharge before printing, it cannot deal with defective discharge in the process of printing. In other words, in drawing a pattern, an unused nozzle among a plurality of nozzles in a droplet discharge apparatus (a nozzle not discharging a droplet) does not perform discharge for a certain period. Therefore, defective discharge may be caused by drying or solidification of a material. As a more highly-volatile solvent is used, a material has higher viscosity due to evaporation of the solvent, and the possibility of defective discharge becomes noticeable.
In order to prevent this, the method in which a moisturizing agent is added to a liquid or the meniscus control method can be used; however, the method in which a moisturizing agent is added may include the case where there is no appropriate moisturizing agent depending on a material. The meniscus control method has a problem in that material viscosity increases due to solvent evaporation over time. Consequently, a change in discharge conditions is caused and the discharge amount or discharge position of the material cannot be controlled precisely. Thus, neither method has reached a fundamental solution.